The present invention pertains to fiber-optical communication systems. More particularly, the present invention relates to a preamplifier used in fiber-optical communication systems that converts optical signals into electronic data, wherein the preamplifier has a high bandwidth, wide dynamic range, and with high stability.
In fiber-optic communication systems, a preamplifier is typically used to convert optical signals received from an optical cable into electronic data. Such a preamplifier generally requires a relative high bandwidth and wide dynamic range. The wide dynamic range generally requires a relatively low noise and high overload.
A prior preamplifier circuit typically uses a resistor to convert a current signal, which is converted from the optical power by the detector, to a voltage signal. The high bandwidth requirement for a preamplifier in fiber-optic communication systems means that the preamplifier circuit needs to have a relatively small resistor. However, a small resistor typically increases the noise of the preamplifier circuit, which can not sense required small input optical power.
On the other hand, a relatively high bandwidth and wide dynamic range preamplifier can be achieved by using a transimpedance feedback amplifier in conjunction with an automatic gain control (AGC) function.
Compare with sole resistor, transimpedance feedback amplifier can be implemented by using a large feedback resistor, which helps to achieve low noise while still achieving high bandwidth. This is due to the fact that a high gain core amplifier can reduce the input impedance of the amplifier. Large feedback resistor can not handle large input optical power, which means AGC is required. AGC means automatically controlling the gain of the transimpedance amplifier according to the input optical power. AGC can help achieve high overload, while still maintaining low noise because at the small input optical power levels, AGC does not function. The gain of the preamplifier can still be high enough to have the low noise feature.
One prior technique for controlling the gain of the transimpedance amplifier is to control the feedback resistance of the transimpedance amplifier. Feedback resistance is reduced as input optical power increases. Changing the feedback resistance, however, affects the impedance and pole frequency at the input node of the transimpedance amplifier. In addition, stability problems arise when the feedback resistance decreases at higher input optical levels. When the resistance of the feedback resistor is decreased to reduce the gain of the preamplifier, the core amplifier should be controlled to maintain the stability of the preamplifier. This is achieved by controlling the bandwidth and the gain of the core amplifier. The bandwidth can be increased, and the gain of the core amplifier is decreased.
FIGS. 1 and 2 show a prior preamplifier 100 that depicts the stability problems and a prior solution in trying to solve the stability problems. As can be seen from FIG. 1, a core amplifier 120 and a feedback resistor 130 form a transimpedance amplifier. A detector 110 is used to convert the input optical signal 105 into electrical signal (i.e., Iin). The resistance of the feedback resistor 130 can vary according to the power of the input optical signal 105.
FIG. 2 shows only the simplified amplifying stage of the core amplifier 120 of FIG. 1, using BiCMOS process. The circuit in FIG. 2 also shows the bandwidth and gain control of the core amplifier 120, which includes a gate-controlled MOSFET 330 connected in parallel with a load resistor 340. The drain of the MOSFET 330 is connected to a power supply rail 310. This means that the MOSFET 330 cannot work in its triode region. Thus, this MOSFET 330 operates as a gate-controlled current source, instead of gate-control resistor. This MOSFET 330 actually controls the transimpedance (Gm) of the core amplifier 120, not the load impedance.
One disadvantage for this structure is that the bandwidth of the core amplifier 120 is difficult to extend. This is due to the fact that the relatively large load resistance (i.e., the load resistor 340) of the core amplifier 120 is required to achieve high gain, thus to meet the bandwidth requirement before AGC active. As described above, the MOSFET 330 actually controls the transimpedance (Gm) of the core amplifier 120, not the load impedance. The load resistance cannot be changed by the configuration shown in FIG. 2. Another disadvantage is, the bias current of the core amplifier 120 may need to vary within a very wide range in order to obtain a widely controlled core amplifier to achieve a wide dynamic range preamplifier 100. This makes the current consumption unacceptable at sensitivity levels. This will be described in more detail below.
For a collector-emitter structure, the gain is calculated as (Gmxc3x97Rc). For the configuration shown in FIG. 2, the transimpedance Gm is controlled, while the load resistor (i.e. Rc 340) is constant. The transimpedance Gm is directly related to the bias current of the amplifying transistor 350. If the gain of the core amplifier 120 is to be controlled widely, the transimpedance Gm should be controlled widely. That is, the bias current should be controlled widely. A suitable dynamic range may, for example, involve core amplifier gain control up to an order of magnitude, that is, a multiplicative range of 10. There is, however, a lower bias current limit provided by the load resistor 340 itself. This lower limit may, for example, be 0.3 mA. These exemplary figures imply a need to provide 3 mA bias current to increase the core amplifier""s gain. In practice, the bias current needs to vary even further, while further current variation may be too great to accept.
In view of the above, a need clearly exists for designing a high bandwidth and wide dynamic range preamplifier with high stability.
The feature of the present invention is to provide a high bandwidth and wide dynamic range preamplifier with high stability.
The detailed feature of the present invention is to control the core amplifier by a dummy transimpedance amplifier and a unit gain buffer while controlling the feedback resistance of the preamplifier such that the preamplifier operates stably over wide dynamic range.
A preamplifier includes a detector to detect an input optical power level and convert the detected input optical power level into an input current. The preamplifier also includes a transimpedance amplifier that converts the input current to voltage signal. A dummy transimpedance amplifier is provided to supply a reference voltage. The dummy transimpedance amplifier has a structure similar to that of the transimpedance amplifier, but it does not receive any input signal. A unity gain buffer is provided to reduce the output impedance of the reference voltage to provide a bias voltage, and to output the bias voltage to both the transimpedance amplifier and the dummy transimpedance amplifier such that the preamplifier operates with high stability, high bandwidth, and wide dynamic range.